The amount of cholesterol present in the blood is known to be related to the risk of coronary artery disease. Cholesterol circulates in the blood predominantly in protein-bound form. The proteins which transport cholesterol are the lipoproteins, which are subdivided into three classes based on their density. The very-low density lipoproteins (VLDL) are triglyceride-rich lipoproteins which are synthesized in the liver and ultimately converted to low-density lipoproteins (LDL), which transport most of the plasma cholesterol in humans. The high-density lipoproteins (HDL) are lipoproteins which are involved in the catabolism of triglyceride-rich lipoproteins, and in the removal of cholesterol from peripheral tissues and transport to the liver. An inverse relationship between serum HDL levels and risk of coronary disease has been established. In particular, if the proportion of serum cholesterol associated with HDL is low, the risk of coronary disease is increased.
In view of the importance of relative serum cholesterol levels in risk assessment and management of atherogenic disease, considerable effort has been spent screening large populations of both normal and high-risk individuals for serum levels of HDL, LDL, as well as total cholesterol and triglycerides. The effectiveness of treatments of high-risk individuals has been monitored by regular testing of serum levels of cholesterol in the various lipoprotein compartments.
One method for specific HDL cholesterol testing is based on the selective precipitation of non-HDL lipoproteins in serum by a polyanionic compound, such as dextran sulfate, heparin, or phosphotungstate, typically in the presence of a group II cation, such as Mg2+, Mn2+, or Ca2+. The specificity and degree of precipitation are dependent on a variety of factors, including the type and concentration of the precipitating reagent. In general, the order of precipitation of serum cholesterol particles, with increasing concentration of polyanion, is VLDL, LDL, and HDL. HDL usually remains soluble at concentrations of heparin or dextran sulfate which completely precipitate lower density particles, although minor apoE species of HDL may be co-precipitated with lower density particles. By selective precipitation of lower density particles, HDL serum cholesterol levels can be determined.
In a typical lipid assay procedure, a small volume of blood is drawn and centrifuged to produce a clear plasma or serum sample fluid. The sample fluid is then aliquoted into several assay tubes, for determination of (a) total serum cholesterol, (b) triglycerides, and (c) HDL cholesterol. The HDL sample is precipitated, as above, and the lower density particles are removed by filtration or centrifugation prior to cholesterol detection. The samples are then reacted with an enzyme mix containing cholesterol esterase, cholesterol oxidase, peroxidase, and a dye which can be oxidized to a distinctly colored product in the presence of H2O2. The tubes may be read spectrophotometrically, and the desired total, HDL and LDL cholesterol values determined.
Despite the accuracy and reliability which can be achieved with the liquid-phase cholesterol assay just described, the assay has a number of limitations for use in widespread screening. First, the method uses a venous blood sample, requiring a trained technician to draw and fractionate the blood sample, and aliquot the treated blood to individual assay tubes. At least one of the sample tubes (for HDL determination) must be treated with a precipitating agent and further processed to remove precipitated material. Although some of these procedures can be automated, analytical machines designed for this purpose are expensive and not widely available outside of large hospitals.
Co-owned U.S. Pat. Nos. 5,213,964, 5,213,965, 5,316,196 and 5,451,370, each of which is incorporated herein by reference, disclose methods and assay devices which substantially overcome many of the above-mentioned problems associated with liquid-assay procedures for measuring serum cholesterol levels. In one embodiment, the device is designed for measuring the concentration of HDL-associated cholesterol in a blood sample also containing LDL and VLDL particles. The device includes a sieving matrix capable of separating soluble and precipitated lipoproteins as a fluid sample migrates through the matrix. A reservoir associated with the matrix is designed to release a soluble precipitating agent, for selectively precipitating LDL and VLDL, as fluid sample is drawn into and through the matrix. This allows HDL separation from the precipitated lipoproteins, based on faster HDL migration through the sieving matrix. The fluid sample, thus depleted of non-HDL lipoproteins, then migrates to a test surface where it is assayed for cholesterol.
The above-referenced devices, while representing an advance over liquid-phase assays, present the possibility of contamination of the flow transport path with the precipitating reagents. Such reagents could interfere with HDL quantification, or with other assay chemistry taking place on other regions of a multi-assay device. The present invention addresses and overcomes these problems.
Further methods and devices for measuring HDL cholesterol in blood samples are disclosed in EP 0408223 and EP 0415298 (Rittersdorf et al.), which describe a continuous assay method carried out on a test strip comprising the following steps and corresponding elements. The blood sample is applied to a separation layer for separating cellular blood constituents. Driven by capillary forces or gravity, the sample flows through a further carrier containing soluble precipitating agents, which, after dissolving in the serum sample, precipitate non-HDL lipoproteins contained in the sample. In a further carrier, the precipitated constituents, above, are filtered from the serum sample to prevent their interference with later HDL quantification. In the same carrier, the sample is transported to a position adjacent the HDL-quantification carrier, and is stored until the HDL quantification step is to be started. Finally, the sample is transferred to an HDL quantification layer, where HDL cholesterol in the serum sample is quantified by an enzymatic reaction.
A disadvantage of this assay design is that the carrier functioning as a reservoir allows migration of the precipitated constituents or soluble reagents into the sample, which can interfere with HDL quantification. In addition, during the storage of the serum sample, HDL can be trapped by adhering to the carrier fibers, precipitating reagents can cause further undesired reactions, and the carrier can become clogged by the drying serum sample.
U.S. Pat. No. 5,135,716 (Thakore) discloses additional devices and methods for HDL quantification in a blood fluid sample. In these devices, the fluid sample flows continuously, though an unbroken path, from an inlet well to a carrier for HDL quantification. Accordingly, the ability to control sample volume entering the HDL test carrier, and to control environmental conditions for the HDL assay, is limited. Nor do the devices provide for simultaneous assay of various analytes from a single fluid sample.
It is therefore the object of the present invention to provide a HDL assay device and method which overcome the above-noted prior art disadvantages.